A Case-Study on the Photophysics of Chalcogen-Substituted Zinc(II) Phthalocyanines.

Singlet dioxygen has been widely applied in different disciplines such as medicine (photodynamic therapy or blood sterilization), remediation (wastewater treatment) or industrial processes (fine chemicals synthesis). Particularly, it can be conveniently generated by energy transfer between a photosensitizer's triplet state and triplet dioxygen upon irradiation with visible light. Among the best photosensitizers, substituted zinc(II) phthalocyanines are prominent due to their excellent photophysical properties, which can be tuned by structural modifications, such as halogen- and chalcogen-atom substitution. These patterns allow for the enhancement of spin-orbit coupling, commonly attributed to the heavy atom effect, which correlates with the atomic number ( Z ${Z}$ ) and the spin-orbit coupling constant ( ζ ${\zeta }$ ) of the introduced heteroatom. Herein, a fully systematic analysis of the effect exerted by chalcogen atoms on the photophysical characteristics (absorption and fluorescence properties, lifetimes and singlet dioxygen photogeneration), involving 30 custom-made ß-tetrasubstituted chalcogen-bearing zinc(II) phthalocyanines is described and evaluated regarding the heavy atom effect. Besides, the intersystem crossing rate constants are estimated by several independent methods and a quantitative profile of the heavy atom is provided by using linear correlations between relative intersystem crossing rates and relative atomic numbers. Good linear trends for both intersystem crossing rates (S1-T1 and T1-S0) were obtained, with a dependency on the atomic number and the spin-orbit coupling constant scaling as Z 0 . 4 ${{Z}^{0.4}}$ and ζ 0 . 2 ${{\zeta }^{0.2}}$ , respectively The trend shows to be independent of the solvent and temperature.

Operando detection and suppression of spurious singlet oxygen in Li-O2 batteries.

The rechargeable lithium air (oxygen) battery (Li-O2) has very high energy density, comparable to that of fossil fuels (∼3600 W h kg-1). However, the parasitic reactions of the O2 reduction products with solvent and electrolyte lead to capacity fading and poor cyclability. During the oxygen reduction reaction (ORR) in aprotic solvents, the superoxide radical anion (O2˙-) is the main one-electron reaction product, which in the presence of Li+ ions undergoes disproportionation to yield Li2O2 and O2, a fraction of which results in singlet oxygen (1O2). The very reactive 1O2 is responsible for the spurious reactions that lead to high charging overpotential and short cycle life due to solvent and electrolyte degradation. Several techniques have been used for the detection and suppression of 1O2 inside a Li-O2 battery under operation and to test the efficiency and electrochemical stability of different physical quenchers of 1O2: azide anions, 1,4-diazabicyclo[2.2.2]octane (DABCO) and triphenylamine (TPA) in different solvents (dimethyl sulfoxide (DMSO), diglyme and tetraglyme). Operando detection of 1O2 inside the battery was accomplished by following dimethylanthracene fluorescence quenching using a bifurcated optical fiber in front-face mode through a quartz window in the battery. Differential oxygen-pressure measurements during charge-discharge cycles vs. charge during battery operation showed that the number of electrons per oxygen molecule was n > 2 in the absence of physical quenchers of 1O2, due to spurious reactions, and n = 2 in the presence of physical quenchers of 1O2, proving the suppression of spurious reactions. Battery cycling at a limited specific capacity of 500 mA h gC-1 for the MWCNT cathode and 250 mA gC-1 current density, in the absence and presence of a physical quencher or a physical quencher plus the redox mediator I3-/I- (with a lithiated Nafion® membrane), showed increasing cyclability according to coulombic efficiency and cell voltage data over 100 cycles. Operando Raman studies with a quartz window at the bottom of the battery allowed detection of Li2O2 and excess I3- redox mediator during discharge and charge, respectively.

Active alcohol consumption is associated with acute-on-chronic liver failure in Hispanic patients.

BACKGROUND: Acute-on-chronic liver failure (ACLF) is a severe clinical entity associated with elevated short-term mortality. We aimed to characterize patients with decompensated cirrhosis according to presence of ACLF, their association with active alcohol intake, and long-term survival in Latin America. METHODS: Retrospective cohort study of decompensated cirrhotic in three Chilean university centers (2017-2019). ACLF was diagnosed according EASL-CLIF criteria. We assessed survival using competing-risk and time-to-event analyses. We evaluated the time to death using accelerated failure time (AFT) models. RESULTS: We included 320 patients, median age of 65.3±11.7 years old, and 48.4% were women. 92 (28.7%) patients met ACLF criteria (ACLF-1: 29.3%, ACLF-2: 27.1%, and ACLF-3: 43.4%). The most common precipitants were infections (39.1%), and the leading organ failure was kidney (59.8%). Active alcohol consumption was frequent (27.7%), even in patients with a prior diagnosis of non-alcoholic fatty liver disease (NAFLD) (16.2%). Ninety-two (28.7%) patients had ACLF (ACLF-1: 8.4%, ACLF-2: 7.8%, and ACLF-3: 12.5%). ACLF patients had a higher MELD-Na score at admission (27 [22-31] versus 16 [12-21], p<0.0001), a higher frequency of alcohol-associated liver disease (36.7% versus 24.9%, p=0.039), and a more frequent active alcohol intake (37.2% versus 23.8%, p=0.019). In a multivariate model, ACLF was associated with higher mortality (subdistribution hazard ratio 1.735, 95%CI: 1.153-2.609; p<0.008). In the AFT models, the presence of ACLF during hospitalization correlated with a shorter time to death: ACLF-1 shortens the time to death by 4.7 times (time ratio [TR] 0.214, 95%CI: 0.075-0.615; p<0.004), ACLF-2 by 4.4 times (TR 0.224, 95%CI: 0.070-0.713; p<0.011), and ACLF-3 by 37 times (TR 0.027, 95%CI: 0.006-0.129; p<0.001). CONCLUSIONS: Patients with decompensated cirrhosis and ACLF exhibited a high frequency ofactive alcohol consumption. Patients with ACLF showed higher mortality and shorter time todeath than those without ACLF.

Photoactive Red Fluorescent SiO2 Nanoparticles Based on Controlled Methylene Blue Aggregation in Reverse Microemulsions.

We present a reverse microemulsion synthesis procedure for incorporating methylene blue (MB), a known FDA-approved type-II red-absorbing photosensitizer and 1O2 generator, into the matrix of hydrophobic-core/hydrophilic-shell SiO2 nanoparticles. Different synthesis conditions were explored with the aim of controlling the entrapped-dye aggregation at high dye loadings in the hydrophobic protective core; minimizing dye aggregation ensured highly efficient photoactive nanoentities for 1O2 production. Monitoring the synthesis in real time using UV-vis absorption allowed tracking of the dye aggregation process. In particular, silica nanoparticles (MB@SiO2 NPs) of ∼50 nm diameter size with a high local entrapped-MB concentration (∼10-2 M, 1000 MB molecules per NP) and a moderate proportion of dye aggregation were obtained. The as-prepared MB@SiO2 NPs showed a high singlet oxygen photogeneration efficiency (ΦΔ = 0.30 ± 0.05), and they can be also considered as red fluorescent probes (ΦF ∼ 0.02, λmax ∼ 650 nm). The distinctive photophysical and photochemical characteristics of the synthesized NPs reveal that the reverse microemulsion synthesis procedure offers an interesting strategy for the development of complex theranostic nano-objects for photodynamic therapy.

Photophysics at Unusually High Dye Concentrations.

The study of the interaction of light with systems at high dye concentrations implies a great challenge because several factors, such as emission reabsorption, dye aggregation, and energy trapping, hinder rationalization and interpretation of the involved photophysical processes. Space constraints induce dye interaction even in the absence of ground state stabilization of dimers and oligomers. At distances on the order of 1 nm, statistical energy traps are usually observed. At longer distances, excited state energy transfer takes place. Most of these factors do not result in ground state spectroscopic changes. Rather, fluorescence phenomena such as inner filter effects, concentration-dependent Stokes' shifts, and changes in quantum yields and decay times are evidenced. Photophysical studies are commonly carried out at high dilution, to minimize dye-dye interactions and emission reabsorption, and in the absence of light scattering. Under these conditions, the physical description of the system becomes rather simple. Fluorescence and triplet quantum yields become molecular properties and can be easily related to ratios of rate constants. However, many systems containing dyes able to fulfill specific functions, whether man-made or biological, are far from being dilute and scattering-free. The photosynthetic apparatus is a paradigmatic example. It is clear that isolation of components allows gathering relevant information about complex systems. However, knowledge of the photophysical behavior in the unaltered environment is essential in most cases. Complexity generally increases when light scattering is present. Despite that, our experience shows that light scattering, when correctly handled, may even simplify the task of unraveling molecular parameters. We show that methods and models aiming at the determination and interpretation of fluorescence and triplet quantum yields as well as energy transfer efficiencies can be developed on the basis of simple light-scattering theories. Photophysical studies were extended to thin films and layer-by-layer assemblies. Procedures are presented for the evaluation of fluorescence reabsorption in concentrated fluid solutions up to the molar level, which are being applied to ionic liquids, in which the emitting species are the bulk ions. Fluorescence reabsorption models proved to be useful in the interpretation of the photophysics of living organisms such as plant leaves and fruits. These new tools allowed the assessment of chlorophyll fluorescence at the chloroplast, leaf and canopy levels, with implications in remote sensing and the development of nondestructive optical methods.

Diagnostic performance of three non-invasive fibrosis scores (Hepamet, FIB-4, NAFLD fibrosis score) in NAFLD patients from a mixed Latin American population.

INTRODUCTION AND AIMS: Several non-invasive scoring systems have been developed and validated worldwide to predict the risk of liver fibrosis in nonalcoholic fatty liver disease (NAFLD). However, information about the performance of these systems in Latin American populations is scarce. Our aim was to evaluate the performance of the Hepamet Fibrosis Score, Fibrosis-4 (FIB-4) and the NAFLD Fibrosis Score (NFS) in a mixed Latin American group of NAFLD patients. METHODS: Clinical, laboratory and liver biopsy data collected from 379 biopsy-proven NAFLD patients from Latin American tertiary health centers were reviewed. Histological fibrosis stages were classified using the Kleiner score. Accuracy was determined, and new fibrosis score thresholds were calculated to better compare the performances of non-invasive tests and to explore their usefulness in excluding fibrosis. RESULTS: The distribution of fibrosis stages among the sample population was as follows: F0 (45%), F1 (27%), F2 (8%), F3 (16%) and F4 (4%). Using modified thresholds, the areas under the ROC curves (AUROC) for Hepamet and FIB-4 (0.73 and 0.74, respectively) to detect significant fibrosis were higher than that of NFS (0.58). However, the AUROCs of the three scores were not significantly different in advanced fibrosis and cirrhosis. To exclude fibrosis, we calculated lower cutoffs than standard thresholds for Hepamet, FIB-4 and NFS with similar performances. CONCLUSION: Thresholds of non-invasive fibrosis scores (Hepamet, FIB-4 and NFS) can be modified to maximize diagnostic accuracy in Latin American patients with NAFLD.

Association between Transverse Sinus Hypoplasia and Cerebral Venous Thrombosis: A Case-Control Study.

BACKGROUND: Hypoplasia of the transverse sinus (TS) is a common anatomical variation. However, the relationship between TS hypoplasia and venous thrombosis has not been studied. We analyzed the hypothesis that TS hypoplasia is a predisposing factor for ipsilateral thrombosis. MATERIALS AND METHODS: We retrospectively evaluated 20 confirmed cases with isolated TS thrombosis and 43 age- and sex-matched controls. TS thrombosis and hypoplasia were diagnosed using both computed tomography and magnetic resonance venography. Hypoplasia was defined as a TS diameter less than 50% of the cross-sectional diameter of the lumen of the distal superior sagittal sinus and by a bony groove ratio less than 1.02. Univariate analysis was performed to evaluate the association between TS hypoplasia and thrombosis. RESULTS: There were a total of 45 hypoplastic TS: 31 (49%) left hypoplastic TS (12 (60%) cases vs 19 (44%) controls (P = .24), and 14 (22%) right hypoplastic TS (9 (45%) cases vs 5 (12%) controls (P = .003). TS hypoplasia was more frequently found in cases (n = 18, 90.0%) than in controls (n = 22, 51.2%; relative risk 1.7, confidence interval [CI] 95% 1.3-2.4, P = .003). Hypoplastic TS and ipsilateral TS thrombosis showed a significant association (P = .002 for right and P = .008 for left TS hypoplasia) with relative risk of 3.8 (95% CI 1.3-10) for right and 7.5 (95% CI 1.1-48) for left hypoplasia. No significant association was found between hypoplastic TS and functional outcome at 30- or 90-day follow-up. CONCLUSION: TS hypoplasia might be a predisposing factor for ipsilateral TS thrombosis, but not for functional outcome.

Steady-State Fluorescence of Highly Absorbing Samples in Transmission Geometry: A Simplified Quantitative Approach Considering Reabsorption Events.

A simplified methodology to acquire steady-state emission spectra and quantum yields of highly absorbing samples is presented. The experimental setup consists of a commercial spectrofluorometer adapted to transmission geometry, allowing the detection of the emitted light at 180° with respect to the excitation beam. The procedure includes two different mathematical approaches to describe and reproduce the distortions caused by reabsorption on emission spectra and quantum yields. Toluene solutions of 9,10-diphenylanthracence, DPA, with concentrations ranging between 1.12 × 10-5 and 1.30 × 10-2 M, were used to validate the proposed methodology. This dye has significant probability of reabsorption and re-emission in concentrated solutions without showing self-quenching or aggregation phenomena. The results indicate that the reabsorption corrections, applied on molecular emission spectra and quantum yields of the samples, accurately reproduce experimental data. A further discussion is performed concerning why the re-emitted radiation is not detected in the experiments, even at the highest DPA concentrations.

Rebleeding and Outcome in Patients with Symptomatic Brain Stem Cavernomas.

PURPOSE: We sought to evaluate the long-term functional outcomes and identify the potential risk factors for rebleeding in patients with brain stem cavernous malformations (BCMs) who presented with hemorrhages and were surgically or conservatively treated and prospectively monitored. METHODS: From January 1990 to July 2015, we included patients with first hemorrhagic episodes secondary to single BCMs. Modified Rankin score (mRS) was used for neurological status assessment. Univariate and multivariate regression statistics were used to identify the risk factors for rebleeding. RESULTS: A total of 99 patients with BCMs hemorrhages were included (59 [59.6%] women, mean age 37± 13 years). As initial treatments, 37 patients (37.4%) underwent surgery and 62 (62.6%) received conservative treatment. The median follow-up was 3.33 years (interquartile range 1.16-7 years; 408.3 patient/years). The rebleeding rate by patient/year was 10% in conservatively treated patients. Deterioration was significantly more frequent in patients with rebleeding (p = 0.0001). At the end of the follow-up, the mRS were favorable in 49 patients (65.3%) without rebleeding, whereas only 8 (33.3%) with rebleeding evolved to favorable outcomes (p = 0.006). Lesion size >18 mm (hazards ratio, HR 3.34, 95% CI 1.54-7.26; p = 0.0001) and ventral location or crossing the brain stem's midpoint (HR 2.5, 95% CI 1.14-5.46; p = 0.022) were associated with a major risk of rebleeding in the univariate analysis, but only a lesion >18 mm remained statistically significant (HR 2.7, 95% CI 1.2-6.21; p = 0.016) in the multivariate analysis. CONCLUSION: A lesion size >18 mm was the principal factor associated with hemorrhage recurrence. The overall functional outcome was good. However, significant morbidity was attributable to rebleeding.

Tuning the concentration of dye loaded polymer films for maximum photosensitization efficiency: phloxine B in poly(2-hydroxyethyl methacrylate).

Fluorescence and singlet molecular oxygen ((1)O2) quantum yields for phloxine B loaded poly(2-hydroxyethyl methacrylate) thin films are determined at dye concentrations from 0.015 to 22 wt%. Fluorescence self-quenching and the fall off of the (1)O2 quantum yield observed above 0.1 wt% are attributed to very weakly interacting close-lying dye molecules acting as energy traps arising from molecular confinement. The maximum singlet oxygen generation efficiency (quantum yield × absorption factor) lies at concentrations around 2 wt%, where fluorescence self quenching amounts to more than 80%. Data are fitted quantitatively by using a quenching radius model involving energy migration and trapping with rQ = 1.2 nm. The present results constitute a proof of concept for the rational design of heterogeneous photosensitizers in general and, particularly, for applications in which the antimicrobial activity of singlet oxygen is central.

Effect of concentration on the formation of rose bengal triplet state on microcrystalline cellulose: a combined laser-induced optoacoustic spectroscopy, diffuse reflectance flash photolysis, and luminescence study.

Laser-induced optoacoustic spectroscopy (LIOAS), diffuse reflectance laser flash photolysis (DRLFP), and laser-induced luminescence (LIL) have been applied in conjunction to the determination of triplet state quantum yields of Rose Bengal (RB) supported on microcrystalline cellulose, a strongly light-scattering solid. Among the three used methods, the only one capable of providing absolute triplet quantum yields is LIOAS, but DRLFP and LIL aid in demonstrating that the LIOAS signal arises in fact from the triplet state and confirm the trend found with RB concentration. The coherence found for the three techniques demonstrates the usefulness of the approach. Observed triplet quantum yields are nearly constant within a limited concentration range, after which they decay strongly due to the generation of inactive dye aggregates or energy trapping centers. When quantum yields are divided by the fraction of absorbed light exciting the dye, the quotient falls off steadily with concentration, following the same trend as the observed fluorescence quantum yield. The conditions that maximize triplet formation are determined as a compromise between the rising light absorption and the decrease of quantum yield with RB concentration.

Clinical spectrum of artery of Percheron infarct: clinical-radiological correlations.

BACKGROUND: The occlusion of the artery of Percheron results in bilateral thalamic and mesencephalic infarctions. In this series, we attempted to classify the subtypes of clinical presentations and long-term prognosis with regards to radiological patterns. METHODS: We sought the clinical and radiological findings of 15 (8 men and 7 women; mean age 48 years) consecutive patients with Percheron artery infarct over 10 years. We classified the clinical symptoms according to the presence of a mental status disturbance (MSD), behavioral amnesic impairment (BAI), aphasia/dysarthria, ocular movement disorders (OMDs), motor deficit, cerebellar signs, and others. The Percheron artery infarct images were classified as bilateral paramedian thalamic with rostral midbrain infarction (BPTRMI), bilateral paramedian thalamic without midbrain infarction (BPTWMI), bilateral paramedian and anterior thalamic with midbrain infarction (BPATMI), and bilateral paramedian and anterior thalamic without midbrain infarction. The outcome was evaluated using a modified Rankin Scale (mRS). RESULTS: OMD and MSD were the most common clinical manifestations in patients with BPTRMI (n = 8). BAI and MSD were the main clinical findings in patients with BPTWMI (n = 6). A patient with BPATMI had a combination of clinical manifestations. After a mean follow-up of 55 months, a good outcome (mRS score ≤ 2) was present in 25% of the patients with BPTRMI, 67% of the patients with BPTWMI, and in 1 patient with BPATMI. CONCLUSIONS: Our findings suggest that it is possible to identify clinical and radiological subgroups of Percheron artery infarct. The long-term follow-up outcome is generally good, except in cases with midbrain involvement.

Evidence on dye clustering in the sensitization of TiO2 by aluminum phthalocyanine.

As previous studies have shown, the photocatalytic reduction of Cr(VI) to Cr(III) in the presence of 4-chlorophenol can be carried out efficiently under visible irradiation using TiO2 modified with hydroxoaluminum-tricarboxymonoamide phthalocyanine (AlTCPc) in spite of the high aggregation tendency of the dye. In the present work, photocurrent and absorption spectra of AlTCPc modified TiO2 films are studied together with absorption and fluorescence of the dye in solution as a function of the concentration of the dye to clarify (a) the role of aggregates and the nature of the species responsible for electron injection into the semiconductor and (b) the reasons why, as reported earlier, the photocatalytic activity is nearly independent of dye loading at constant TiO2 mass. Results are consistent with the presence of AlTCPc clusters with similar properties both on the TiO2 surface and in H2O-DMSO solution. The actual photoactive species is the monomeric dye electronically coupled to the semiconductor. Monomer concentration depends only slightly on AlTCPc analytical concentration, in a way similar to surfactant monomers in micellar equilibrium, thus explaining the independence of photocatalytic activity on dye concentration.

Phloxine B as a probe for entrapment in microcrystalline cellulose.

The photophysical behaviour of phloxine B adsorbed onto microcrystalline cellulose was evaluated by reflectance spectroscopy and laser induced time-resolved luminescence in the picosecond-nanosecond and microsecond-millisecond ranges. Analysis of the absorption spectral changes with concentration points to a small tendency of the dye to aggregate in the range of concentrations under study. Prompt fluorescence, phosphorescence and delayed fluorescence spectral decays were measured at room temperature and 77 K, without the need of sample degassing because cellulose protects triplet states from oxygen quenching. In all cases, spectral changes with time and lifetime distribution analysis were consistent with the dye coexisting in two different environments: dyes tightly entrapped between polymer chains in crystalline regions of cellulose showed longer fluorescence and phosphorescence lifetimes and more energetic triplet states, while dyes adsorbed in more amorphous regions of the support showed shorter lifetimes and less energetic triplet states. This behaviour is discussed in terms of the different dye-support interactions in both kinds of adsorption sites.

Diversity regained: Precautionary approaches to COVID-19 as a phenomenon of the total environment.

As COVID-19 emerged as a phenomenon of the total environment, and despite the intertwined and complex relationships that make humanity an organic part of the Bio- and Geospheres, the majority of our responses to it have been corrective in character, with few or no consideration for unintended consequences which bring about further vulnerability to unanticipated global events. Tackling COVID-19 entails a systemic and precautionary approach to human-nature relations, which we frame as regaining diversity in the Geo-, Bio-, and Anthropospheres. Its implementation requires nothing short of an overhaul in the way we interact with and build knowledge from natural and social environments. Hence, we discuss the urgency of shifting from current to precautionary approaches to COVID-19 and look, through the lens of diversity, at the anticipated benefits in four systems crucially affecting and affected by the pandemic: health, land, knowledge and innovation. Our reflections offer a glimpse of the sort of changes needed, from pursuing planetary health and creating more harmonious forms of land use to providing a multi-level platform for other ways of knowing/understanding and turning innovation into a source of global public goods. These exemplary initiatives introduce and solidify systemic thinking in policymaking and move priorities from reaction-based strategies to precautionary frameworks.

Diversity lost: COVID-19 as a phenomenon of the total environment.

If we want to learn how to deal with the COVID-19 pandemic, we have to embrace the complexity of this global phenomenon and capture interdependencies across scales and contexts. Yet, we still lack systematic approaches that we can use to deal holistically with the pandemic and its effects. In this Discussion, we first introduce a framework that highlights the systemic nature of the COVID-19 pandemic from the perspective of the total environment as a self-regulating and evolving system comprising of three spheres, the Geosphere, the Biosphere, and the Anthroposphere. Then, we use this framework to explore and organize information from the rapidly growing number of scientific papers, preprints, preliminary scientific reports, and journalistic pieces that give insights into the pandemic crisis. With this work, we point out that the pandemic should be understood as the result of preconditions that led to depletion of human, biological, and geochemical diversity as well as of feedback that differentially impacted the three spheres. We contend that protecting and promoting diversity, is necessary to contribute to more effective decision-making processes and policy interventions to face the current and future pandemics.

Silicon nanoparticle photophysics and singlet oxygen generation.

The effect of molecular oxygen and water on the blue photoluminescence of silicon nanoparticles synthesized by anodic oxidation of silicon wafers and surface functionalized with 2-methyl 2-propenoic acid methyl ester is investigated. The particles of 3 +/- 1 nm diameter and a surface composition of Si(3)O(6)(C(5)O(2)H(8)) exhibit room-temperature luminescence in the wavelength range 300-600 nm upon excitation with 300-400 nm light. The luminescence shows vibronic resolution and high quantum yields in toluene suspensions, while a vibronically unresolved spectrum and lower emission quantum yields are observed in aqueous suspensions. The luminescence intensity, though not the spectrum features, depends on the presence of dissolved O(2). Strikingly, the luminescence decay time on the order of 1 ns does not depend on the solvent or on the presence of O(2). To determine the mechanisms involved in these processes, time-resolved and steady-state experiments are performed. These include low-temperature luminescence, heavy atom effect, singlet molecular oxygen ((1)O(2)) phosphorescence detection, reaction of specific probes with (1)O(2), and determination of O(2) and N(2) adsorption isotherms at 77 K. The results obtained indicate that physisorbed O(2) is capable of quenching nondiffusively the particle luminescence at room temperature. The most probable mechanism for (1)O(2) generation involves the energy transfer from an exciton singlet state to O(2) to yield an exciton triplet of low energy (<0.98 eV) and (1)O(2). In aqueous solutions, excited silicon nanoparticles are able to reduce methylviologen on its surface.

Trapping of Rhodamine 6G excitation energy on cellulose microparticles.

Rhodamine 6G (R6G) was adsorbed on cellulose microparticles and fluorescence quantum yields and decays were measured as a function of dye loading. Though no spectroscopic evidence of dye aggregation was found, a noticeable decrease of quantum yield--after correction for reabsorption and reemission of fluorescence--and shortening of decays were observed at the highest loadings. These effects were attributed to the dissipation of the excitation energy by traps constituted by R6G pairs, leading to static and dynamic quenching produced by direct absorption of traps and non-radiative energy transfer from monomers, respectively. Regarding the nature of traps, two extreme approaches were considered: (a) equilibrium between monomers slightly interacting in the ground state and (b) randomly distributed monomers located below a critical distance (statistical traps). Both approaches accounted quantitatively for the observed facts. The effect of energy migration was evaluated through computational simulations. As the concentration of traps could only be indirectly inferred, in some experiments an external energy transfer quencher, Methylene Blue, was coadsorbed and the results were compared with those obtained with pure R6G.

Excitation energy transfer and trapping in dye-loaded solid particles.

The photophysics of several systems composed of a single dye or pairs of dyes attached to solid particles has been studied in the dry solid state at high dye concentrations taking into account light scattering and inner filter effects. Interaction among dye molecules and singlet-singlet energy transfer are relevant in these conditions, as has been demonstrated for pairs of dyes with suitable spectral overlap. For single dyes, after correction for radiative energy transfer, fluorescence quenching is observed as the surface concentration increases. This effect is explained by two different trapping models. Irrespective of the nature of the traps, concentration quenching may be of static (trap absorption) and dynamic (energy transfer) nature. The unraveling of energy trapping mechanisms is a key to the development of efficient photoactive solid materials.